Para-xylene is a valuable chemical feedstock which may be derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range of 10 to 32 wt. % ethylbenzene (EB) with the balance, xylenes, being divided approximately 50 wt. % meta and 25 wt. % each of para and ortho.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation, although this is a costly operation. Ortho-xylene may be separated by fractional distillation, and is so produced commercially. Para-xylene may be separated from the mixed isomers by fractional crystallization, selective adsorption (e.g., the Parex process), or membrane separation.
As commercial use of para-xylene has increased, combining physical separation with chemical isomerization of the other xylene isomers to increase the yield of the desired para-isomer has become increasingly important. However, since the boiling point of ethylbenzene is very close to those of para-xylene and meta-xylene, complete removal of ethylbenzene from the C.sub.8 aromatic feed by distillation is impractical. Hence an important feature of any commercial xylene isomerization process is the ability convert ethylbenzene in the feed to useful by-products while simultaneously minimizing any conversion of xylenes to other compounds.
One commercially successful xylene isomerization process is described in U.S. Pat. No. 4,899,011 in which a C.sub.8 aromatic feed, which has been depleted in its para-xylene content, is contacted with a two component catalyst system. The first catalyst component selectively converts the ethyllbenzene by deethylation, while the second component selectively isomerizes the xylenes to increase the para-xylene content to a value at or approaching the thermal equilibrium value. The first catalyst component comprises a Constraint Index 1-12 molecular sieve, such as ZSM-5, which has an ortho-xylene sorption time of greater than 50 minutes based on its capacity to sorb 30% of the equilibrium capacity of ortho-xylene at 120.degree. C. and an ortho-xylene partial pressure of 4.5.+-.0.8 mm of mercury, whereas the second component comprises a Constraint Index 1-12 molecular sieve which has an ortho-xylene sorption time of less than 10 minutes under the same conditions. In one preferred embodiment, the first catalyst component is ZSM-5 having a crystal size of at least 1 micron and the second catalyst component is ZSM-5 having a crystal size of 0.02-0.05 micron. Each catalyst component also contains a hydrogenation component, preferably a platinum group metal.
An improvement over the process of U.S. Pat. No. 4,899,011 is described in U.S. Pat. No. 5,689,027 in which the first catalyst component in the two component system is pre-selectivated by coking, or more preferably by deposition of a surface coating of silica, to increase its ortho-xylene sorption time to greater than 1200 minutes under the same conditions as cited in the '011 patent. Using such a system it is found that high ethylbenzene conversion rates can be achieved with significantly lower xylene losses than obtained with the process of the '011 patent.
An object of the present invention is to further reduce the xylene losses obtainable with existing two component xylene isomerization processes such as those described in U.S. Pat. Nos. 4,899,011 and 5,689,027.